A wide variety of organizations, such as military, aerospace, aeronautical, nautical, and telecommunications industries use dual polarized electromagnetic waves as a means of transmitting and receiving information. In order to enable the transmission and reception of dual polarized waves, it is necessary to employ dual polarized antennas. In these industries there are many known types of dual polarized antennas, some of which are discussed below.
It has long been recognized in the study of electromagnetics that a circularly polarized wave may be thought of as being comprised of two orthogonal, linearly polarized waves, i.e., a vertically polarized wave and a horizontally polarized wave. Using what has become a well-known concept, Walter van B. Roberts obtained a patent for a circularly polarized antenna comprised of a vertically polarized dipole element and a horizontally polarized loop element. See U.S. Pat. No. 2,174,353 xe2x80x9cTransmission of Waves with Rotary Polarization,xe2x80x9d(1939), the contents of which are hereby incorporated by reference. This patent took advantage of the fact that a dipole and a loop are complementary antenna pairs. They have exactly the same radiation pattern, but have cross-polarized fields.
In the years since the Roberts"" patent issued, antenna engineers have continued to be fascinated by the idea of using a loop-dipole combination antenna as a means of constructing a circularly polarized antenna. See e.g., George H. Brown et al., Circularly-Polarized Omnidirectional Antenna, RCA Review, 259-69 (1947); U.S. Pat. Nos. 2,324,462 (Leeds et al. 1943); U.S. Pat. No. 2,953,782 (Byatt 1960); U.S. Pat. No. 3,474,452 (Bogner 1969); and U.S. Pat. No. 3,665,479 (Silliman 1972), and more recently, two Japanese patents, JP 59012601 (1984) and JP 1100851 (1990), each of the contents of which are hereby incorporated by reference. Although the combination of a loop and a dipole may appear to be straightforward to implement, there are, in fact, many systemic limitations that reduce the functionality of this combination. For example, loop antennas are characterized by a very narrow bandwidth, which results in a lossy antenna that is useful only over a very small frequency band. In addition, parasitic interference in the feeding network used to power the loop-dipole combination can be difficult to overcome. Moreover, achieving the proper balance in amplitude and phase over a required bandwidth is a nontrivial exercise.
In the past, engineers have attempted to compensate for these design shortcomings in a variety of ways, including replacing the loop and/or dipole with another type of antenna element or varying the physical location of the loop relative to the dipole. An article published in 1975 illustrates an example of replacing the loop element with alternative elements, in this case multiple tilted or bent half-wave electric dipoles. See Spencer, T., The Omnidirectional Circular Antenna Array, International Conference on Antennas for Aircraft and Spacecraft, 178 (1975), the contents of which are hereby incorporated by reference. Although this configuration mitigated some of the limitations of the loop-dipole combination, the resulting antenna was impractical for many uses because of its large physical cross-section.
Antenna engineers have alternatively substituted a slot on a hollow conductive cylinder for the loop in the dipole-loop antenna and have thus used a slot-dipole combination to create a circularly polarized antenna. In order for a slot to be an effective radiating element, many of the prior art slot antennas"" designs, which required back-cavities of about a quarter wavelength deep or half wavelength in cross-sectional circumference, resulted in prohibitively large antenna elements when the slot and dipole were collocated in the same physical structure. The following U.S. Patents are directed toward a dipole-slot antenna: U.S. Pat. No. 4,710,775 to Coe; U.S. Pat. No. 4,839,663 to Kurtz; U.S. Pat. No. 4,451,829 to Stuckey, Jr. et al.; U.S. Pat. No. 5,021,797 to Dienes; and U.S. Pat. No. 5,426,439 to Grossman, each of the contents of which are hereby incorporated by reference.
An additional example of a circularly polarized antenna is the helix. A helical antenna is, in essence, superposed electrical dipole and loop, which is a magnetic dipole. This superposition can create a circularly polarized antenna. Most helices are used in the axial mode to radiate end-fire patterns. In theory, a helix operating in the normal mode can produce a circularly polarized field with an omni radiation pattern normal to the helix""s axis. JOHN D. KRAUS, ANTENNAS, 173 (1950). The drawback of a normal-mode helix, which is inherent because of its design, is its extremely high radiated xe2x80x9cQxe2x80x9d. A high Q radiated value for an antenna is indicative of low efficiency and of a narrow bandwidth. Harold A. Wheeler, Helical Antenna for Circular Polarization, Proceedings of the Institute of Radio Engineers, 1484, 1487 (1947), the contents of which are hereby incorporated by reference.
An additional alternative, which is likewise used in place of a loop-dipole combination antenna, is cross-dipoles. Although cross-dipoles are an efficient antenna, they are end-fire. Using multiple cross-dipoles in a circular array to form an omni-directional antenna is one way of overcoming this end-fire limitation. The problem with using multiple cross-dipoles is the amount of physical space required to implement the design, which is often prohibitive.
In addition to varying the types of antenna elements used to construct circularly polarized antennas, engineers have also varied the physical locations of the loop or dipole elements relative to each other in an attempt to improve the antenna performance. For example, Leeds et al. located a dipole in the center of a loop in U.S. Pat. No. 2,324,462 (1943). See also U.S. Pat. No. 2,953,782 issued to Byatt (1960). Bogner and Silliman placed dipoles on the outer edge of the loops in their U.S. Pat. Nos. 3,474,452 (1969) and 3,665,479, (1972) respectively. Each of these antennas was impractical because the size was too large for many applications and because the feed lines used for receiving and transmitting power caused interference.
The loop-dipole antennas and variations thereof disclosed in the prior art are large in size, cumbersome in form, and inefficient in performance. Many of these prior art antennas experience feed line and supporting structure interference problems, resulting in performance degradation. The present invention overcomes many of these drawbacks. All embodiments disclosed herein use the same physical components to perform both the electric dipole and the loop antenna functions. These embodiments maximizes the slot antenna gain for a given cross-section size, while maintaining a simple donut-shaped radiation pattern for both orthogonal polarizations. Feed line and supporting structure interference and stray radiation problems are also eliminated in the present invention by housing them inside of a hollow shaft, which can be included in the invention. The present inventive antenna is useful as a longitudinal antenna or in linear array applications.
In one embodiment, the antenna element is comprised of two substantially cylindrical members of nearly equal length, wherein each member is further comprised of a capacitively loaded axial slot. The exterior of this embodiment acts as a dipole, wherein the slots are short-circuited at one end and form an open circuit at the other end. An alternative embodiment is comprised of two substantially polygonal members of nearly equal length, wherein each member is further comprised of a capacitively loaded axial slot. The exterior of this embodiment acts as a dipole, wherein the slots are short-circuited at one end and form an open circuit at the other end. Yet another embodiment of the present invention comprises more than one slot spaced evenly on the peripheral surface of the antenna.
Another embodiment is comprised of core material with high permeability located in the center of the antenna element. In addition, the antenna element of the present invention could be configured in an array by using a plurality of antennas designed in accordance with the principles disclosed herein. Additional embodiments of the antenna include design aspects directed toward increasing the bandwidth and efficiency of the slot portion of the antenna, while maintaining a relatively symmetrical donut-shaped radiation pattern.